du: util for "disk usage" added

[odeke-em]

This PR addresses #100 by only adding du.

Added a utility analogous to UNIX util "du".
A couple of differences though:

• drive's du prints breadth first ie level by level.
• drive's du prints the bytes of every single file, not just
  directories.

[akomakom]

@odeke-em is there any documentation on this feature anywhere? This PR says "not just directories", implying that it handles directories, but I've found no equivalent to du -s (directory size total) in this utility:

$ drive du temp
0            /temp/3
0            /temp/2
0            /temp/1

$ drive du -no-prompt -recursive temp
0            /temp/3
0            /temp/2
0            /temp/1
3801088      /temp/3/test2.txt
4456448      /temp/3/test1.txt
4730880      /temp/2/test1.txt
6971392      /temp/2/test2.txt
7077888      /temp/1/test2.txt
6553600      /temp/1/test.txt

$ drive du -no-prompt -recursive -directories temp
0            /temp/3
0            /temp/2
0            /temp/1

Thanks!

Workaround:

Single sum at this level: (like du -s)
drive du -no-prompt -recursive | awk '{s+=$1} END {print s}'

Detailed usage for each sub-item: ( like du -s * but without the total )
drive ls | while read line ; do echo "$(drive du -no-prompt -recursive "${line/\//}" | awk '{s+=$1} END {print s}') $line" ; done